The efficiency of a gas turbine is improved together with an increase in combustor outlet temperature or turbine inlet temperature. The combustor outlet temperature of the current gas turbine reaches 1500° C. The temperature of the surface of a gas turbine blade exposed to the high-temperature combustion gas exceeds a limit temperature of a heat-resistant alloy used, which requires cooling of the gas turbine blade.
Air extracted from a compressor is supplied to a cooling channel formed in the gas turbine blade, and subjected to convection cooling. The air is injected from the cooling channel to the surface of the gas turbine blade via through holes set in the blade surface and flows over the blade surface to perform film cooling, thereby suppressing an increase in temperature of the gas turbine blade to decrease the temperature to the limit temperature or less. However, there are some positions of the blade where film cooling holes are difficult to be effectively arranged due to the restrictions on the shape and manufacturing of the blade, and the like.
In the tip of the gas turbine blade, a combustion gas might leak in clearance between the blade tip and an inner surface of a casing in the radial direction, leading to a loss in work of the turbine. In order to reduce the loss, the clearance is designed to be minimum. Upon start-up of the gas turbine, however, a difference in thermal expansion between the gas turbine blade and the casing might be caused due to a difference in temperature between the blade and casing generated in stopping of the turbine, so that the blade tip might be brought into contact with the casing to be worn. Thus, the tip of the gas turbine blade generally has a partition for isolating the cooling channel formed in the blade from the outside and a blade portion extending from the partition in the direction of the outer diameter to form a tip end wall, which serves as a wear allowance.
The tip end wall, however, is spaced apart from the cooling channel formed in the gas turbine blade, which makes it difficult to cool the blade tip even though the film cooling holes are provided from the cooling channel toward the blade tip. In particular, the surface of a space between the adjacent holes is very difficult to be cooled. Although the clearance between the blade tip and the casing in the radial direction is designed to be minimum, another clearance might be generated with the progress of the wear of the tip end wall. When the combustion gas invades the inner surface side of the tip end wall, the inner surface of the tip end wall would also be exposed to the combustion gas, causing damage to the tip end wall due to oxidation or the like.
In contrast, Japanese Unexamined Patent Publication No. 2005-54799 (see FIG. 4) (Patent Document 1) discloses a structure which includes a reinforcement disposed on an inner surface side of a tip end wall of each blade to thereby suppress the generation of local stress in forming film cooling holes at the tip end wall (see Patent Document 1).
The technique disclosed in Patent Document 1 expects the outer surface of the tip end wall to be cooled. However, the reinforcements are uniformly provided over its inner surface side of the tip end wall. Thus, the thickness of the tip end wall is increased, resulting in an increase in thermal capacity of the tip end wall, which is disadvantageous from the viewpoint of suppressing the increase in temperature of the inner surface. When the reinforcements are provided in a cycle corresponding to positions of the film cooling holes, a superficial area of the inner surface of the blade is increased to promote the heat transfer from the inner surface. Thus, the difference in temperature between the inner and outer surfaces of the tip end wall can be increased to generate the thermal stress.
When the film cooling holes are provided toward the tips of the blades, the cooled air is not brought into contact with the outer surface of the blade between the adjacent holes, making it difficult to uniformly cool the tip end wall from a leading edge of the blade to a trailing edge thereof. Techniques for resisting higher temperatures need to be developed in the future.
Accordingly, it is an object of the present invention to provide a gas turbine blade that suppresses the generation of a local stress by provision of cooling holes, while suppressing a difference in temperature between inner and outer surfaces of the tip end wall of the blade.